1. Field of the Invention
This invention pertains to the field of multichannel switching systems remotely controlled from a central control unit, in particular to satellite-borne relay systems with a plurality of ground stations each of which is itself a telephone exchange, as described in U.S. Pat. No. 3,842,350 of Gross (U.S. Cl. 325/4).
2. Summary of the Prior Art
Indicating the status of a single communicating device to however large a number of interested persons (or devices) is simple; the "abc" or "v" wheels of coastal radio stations require only a single communication channel no matter how many receivers hear them. To report the status of a large number of communicating devices to a single control unit is a quite different problem; it is exacerbated if the method involves interrogation by the control unit over a channel causing appreciable time delay and reply over a channel causing a like delay. The roughly fifty-thousand mile round trip to a synchronous terrestrial satellite and back will itself impose a transit-time delay of about 0.27 seconds, apart from any delays in the relay device itself. Thus a double round trip for interrogation and reply would impose a delay of about 0.6 seconds.
The applicant has not searched the extremely voluminous and crowded telephone art. He is generally familiar with three approaches to the problem of reporting of multiple status data. Comsat Corporation's SPADE system does not not have a central control, and provides communication among a plurality of stations by time sharing--that is, assignment of a given time period for communication between a specific pair of stations. In another instance, data from ground-based sensors to a surveying type of satellite is repeated so that overlap will not cause loss of data; what is lost once by overlap with other data will be caught on a later repetition. Comsat Corporation's MARISAT also relies upon repetition to eliminate the consequences of overlap from various sources.
U.S. Pat. No. 3,842,350 of Gross, assigned to the assignee of the present application, in column 12, line 54, through column 14, line 42, describes how data of any kind stored in a given channel interface unit (therein designated as a trunk access unit or acronym TAU) may be read out in its due sequence, and the channel be skipped if there is no data to be read out. It does not provide for intermingling transmission of status messages with other service messages, nor for initiating the transmission of status messages and causing this to continue independently (while other service message traffic may be handled) until another order to stop the transmission of status messages is received.
None of the prior art known to the applicant teaches the transmission of status data by automatic time sharing with other types of service messages responsively to an initiation signal, continuing without further command until a second termination signal is received; nor a method for causing reports from different locations to arrive in close but nonoverlapping time sequence at a central control unit despite long transit times for signals between the locations and the central control unit.